Receiver for light-flash signaling system



June 1960 F. FRUENGEL 2,941,084"

RECEIVER FOR LIGHT-FLASH SIGNALING SYSTEM 7 Filed Dec. 12, 1952 2Sheets-Sheet 1 INVEN TOR. Frmw: FRUENG'EL Am/ZNQL June 14, 1960 F.FRUENGEL 2,941,084

RECEIVER FOR LIGHT-FLASH SIGNALING SYSTEM Filed Dec. 12, 1952 2Sheets-Sheet 2 FIG. 3

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United States Patent RECEIVER FOR LIGHT-FLASH SIGNALING SYSTEM FrankFruengel, 251 Tinsdaler Kirchenweg, Hamburg-Rissen, Germany Filed Dec.12, 1952, Set. No. 325,511

Claims priority, application Germany Jan. 2, 1952 8 Claims. (Cl.250-214) The present invention relates to a receiver for light flashsignals and particularly to an electronic apparatus for receiving andconverting into distinctly visible or audible signals the light flashessent out by an electric impulse lamp as described and claimed in mycompanion application, Serial No. 325,512, filed December 12, 1952 whichmatured on June 5, 1956, into Letters Patent 2,749,482. The systemreferred to, while it can be used for communication at relatively shortdistances between moving stations anywhere, is especially destined toserve as safety means in road traflic and may be termed overtake signalsystem, since it enables motor cars on the road to signal and therebycaution vehicles, especially trucks, ahead before overtaking.

It is an object of this invention to provide a receiver which isparticularly adapted to receive the light impulses of spark dischargecharacter, as transmitted by a cooperating impulse lamp, and to convertthem into readily perceptible signals. Therefore light flashes of sparkdischarge character or spark discharge flashes to be understood asclearly defining in this specification and the claims such light flashesas having each one the extremely short duration between 10' and secondand a steepfronted wave shape giving rise to a momentary increase inluminous flux at a rate of more than 10 lumens per 1 microsecond.

Another object is to provide a light flash receiver which is sosensitive and reliable that all advantages rendering the light flashsignaling system superior to acoustic systems are fully utilized, andthis chiefly on account of the fact that the receiver responds only tothe light flashes transmitted to it and does not react upon lightimpressions that are happening in nature or are incidentally made bymen.

For carrying the invention into effect, the receiver for the particularlight flashes sent out by the impulse lamp is provided with a pickup orconverging lens before a blue light filter upon which the light of theimpulses falls. Behind the filter there is a phototube having preferablya cesium-antimony cathode and being particularly sensitive to light inthe blue and violet region of the spectrum, because it is especiallysuch light that emanates fiom an electric spark being the light sourceof the electric impulse lamp serving as transmitter. The light filterprovided serves to retain red and yellow light which would harm thephototube by heating when direct sunlight would fall on it. I

The voltage impulses caused in the phototube by the incident lightflashes are conducted to electronic amplifier stages, whereby a circuitof properly related capacitance and resistance, according to thisinvention, admits only impulses of a duration of less than about 5 X 10second for amplification, whereas impulses of longer duration canpenetrate only weakened or not at all. This fact renders the receiverinsensitive. to strange light impulses "ice p in receiving only thelight impulses sent out by the transmitter.

so tuned that preferably only frequencies between 100 incidentallyoccurring in its vicinity, and thus very reliable and 1,500 kilocyclesare amplified, the voltage impulses reach the grid of the first tube ofa relaxation type oscillator'having two tubes. This relaxation circuitis so designed that during the relaxation period a relatively high anodecurrent flows through the second tube of the oscillator, which suflicesto cause a glow lamp to light, or a relay to respond. The relay in turnmay close the circuit of an incandescent lamp or a buzzer, which thengives a clearly perceptible signal.

Since the relay actuated by the anode current in accordance with thisinvention, can be only very small, its contacts are able to switch onlyminute currents; therefore a second relay in a separate circuit isprovided to be actuated by the relay in the anode circuit. This secondrelay can be retarded in its action by an auxiliary condenser and can soeffect that after each energizing impulse the indicating lamp or thebuzzer remains on for about one second. If in such arrangement the lightimpulses of the transmitting impulse lamp occur at a higher rate thanone per second, the second relay will remain continuously closed for thewhole period the light flash transmitter is in action and thus theindicating lamp or buzzer will also be on continuously throughout suchperiod.

Since the light flash receiver as provided by this invention isprimarily intended for operation on motor trucks or the like, the pickuplens must be accessible for the light flashes sent out by the carbehind, and therefore must be located at the rear of vehicles of abovetype where it is difficult to protect the receiver from weather andother external influences. In order to obviate the danger of damage,this invention provides that in certain cases only lens and phototube asa separate unit are mounted at the rear end of the vehicle and that thereceiver is placed remote therefrom in a safe location near the driverscab. The connection between both units is then in the form of a cable oflow capacitance such as a coaxial cable.

For a clearer understanding of the foregoing and further novel featuresof the present invention, some preferred embodiments will now bedescribed by way of example in connection with the accompanying drawingsin which:

Fig. 1 represents a complete circuit diagram of a light flash receiveraccording to this invention;

Fig. 2 illustrates a modified relay circuit for actuating the outputsignaling means;

Fig. 3 shows a partial circuit diagram indicating the manner ofconnecting the pickup or phototube unit with the amplifier unit in aninstallation where these two units are remote from each other;

Fig. 4 shows another partial circuit diagram indicating modified meansfor connecting the two units; and

Fig. 5 illustrates still another modification of the connections shownin Figs. 3 and 4.

Referring to the circuit diagram Fig. 1, there can be seen at the leftthe phototube 1 which is reached by the incident light impulses of thetransmitter through a converging lens 29 and a blue filter 30. Thesesiumantimony cathode 1k of the phototube 1 has its optimum sensitivityof about 7 quanta per electron in the blue and violet region of thespectrum, which is the spectral region of an electric spark forming thelight source of the transmitting impulse lamp. The incident light fiashcauses a negative voltage impulse on the anode 1a otfl the phototube 1.-This. negative voltage impulse'fpro- Patented June 14, 1960 gressesthrough the anode connection and through the 7 coupling condenser 2 ofabout 50 micromicrofarad to the grid 4g of the first amplifier tube 4. Agrid leak resistor 3 of about 0.1 megohm is connected to a junctionpoint in the connection between condenser 2 and grid 4g, The combinationof the selected capacitance (50 micromicrofarad) and resistance (:1megohm) results in a time constant of 5() 10- l0 =5 10* second. Thistime constant has the'efiect that substantially only voltage impulses ofa duration of less than 5 l0- second can reach the grid of 4g of theamplifier tube 4. Longer lasting impulses are markedly weakened orcannot pass at all. By exact design calculation of the constants in thecircuitshown and by proper choice of components particularly thecapacitive coupling means, it has been achieved that the receiverresponds to, and amplifies only light flashes lying in the blue andviolet spectral region and having a steep gradient, i.e., beingsuchlthat their luminous'fiux rises at a rate steeper than 100 lux permicrosecond. For light impulses with such steep gradient, impulses assent out by the impulse lamp described in my companion patentapplication previously referred to, the receiver is particularlysensitive. The anode resistors 10, 11, 12 of the first threeamplifiertubes 4, 5, 6 have relatively low resistance, only a few thousand ohms.Thisis necessary in order to'obtain uniform amplification of thehigh-frequency voltage impulses in spite of the fact that the tubecapacitances are bridged by these resistors. Amplification of onlyhigh-frequency impulses, or impulses of steep-fronted wave shape, isimportant, because no benefit can be had from amplification oflowfrequency voltage changes. On the contrary, such amplification wouldrender the receiver less disturbancepro0f, since it could then react tolight effects other than thetransmitted flashes. The restriction to onlyhighfrequency amplification necessitates operation withlow amplificationfactors of the order of m 30 per stage. In the frequency range between100 to 1,500 kilocycles,

amplification is then substantially linear. At higher fre-' quencies,the amplification decreases and is at 3,000 kilocycles already 2' to 3neper lower than maximum amplification. 7 i The voltage impulse, whichoccurs negative at the anode 1a of the phototube 1, becomes positivebehind the first amplifier tube 4, negative behind the second amplifiertube 5, and again positive behind the third amplifier tube 6. The stages4 and 5, and 5 and 6, respectively, are capacitively coupled by couplingcondensers 8 and 9, respectively. The tube 6 operates with a high anodecurrent. Its cathode 6k is connected with cathode 7k of tube 7 and isnot bridged by a condenser. Now, when a negative voltage impulse arrivesat grid 6g of tube'6, the anode current of this tube Willdecrease andconsequently the cathode 6k becomes more negative. Simultaneouslytherewith, tube 7 becomes conductive due to the fact that a positivevoltage impulse reaches its grid 7g. 'In'consequence thereof, thevoltage at the cathode resistor 13 is now raised more than it had beendecreased by the current drop at tube'6, and this because a stage ofamplification lies therebetween. Increase of voltage on cathode resistor13 renders the cathode 6k more positive and the grid 6g in relationthereto more negative, whereby this negative potential is additive tothe original small negative impulse received from the second amplifiertube 5. Thus, the negative charge of grid 6g increases by cumulativeaction until the tube 6 is completely cut oil, whereas tube 7 becomesfully conductive. Return to the original condition occurs then inaccordance with the time constant established by grid condenser 14 (50micromicrofarad), interposed between tubes 6 and 7, and the grid leakresistance 15 (1 megaohm) inSOX 10- X10 =50X 10* second; During thisrelaxation period of about second, there flows through tube 7arelatively high anode current (about 10 milliamp'eres) whichis'sufijcient to energize relay coil 16, or to light a glow lamp whichmay be inserted instead of the relay. The relaxation type oscillatorcircuit just described is so sensitive that an impulse of about $4 voltand a duration of 10- second applied to grid 6g is able to start arelaxation cycle of the tubes 6, 7.

Since the energy that can be drawn from the anode output of tube 7 forenergizing relay coil 16 is very low, its contacts 17 can handle onlysmall currents which in some cases are not suflicient to give positivelyperceptible signals. For example, if it is desirable to light one orseveral indicating lamps, including a return-signal lamp for giving aconfirming signal to the car in rear, or to sound a'buzzer, it has beenfound advisable to interpose another type of relay circuit between thepoints A, B and C in the anode connection. Such a circuit, as indicatedin Fig. 2, includes the relay coil 16 which in this case actuates anormally closed contact 18 interposed in a battery circuit whichtherefore normally serves to energize a second relay coil 19 which, whenenergized, holds its contact 20 open. 'The contact 20 is inserted in abattery circuit providing current for the signaling means shown in thefigure as an indicating lamp 22, a buzzer 23 and a return-signal lamp37. It willbe noted that when relay 16 is energized, it opens contact 18and de-energizes thereby the second relay coil 19, which in turnreleases its armature and closes thereby contact 20 and the circuit forthe signaling means. This second relay coil 19 can be considerablylarger and its contacts can handle currents which are adequate toenergize most effective signaling means. In the arrangement justdescribed it is possible to delay the action of the second relay byinterposing a condenser 21 (about 4 microfarads) in the auxiliarycircuit and achieve thereby that this relay remains closed for about onesecond after each impulse from the relaxation circuit, and thus causesthat also a signal lamp lights for such a prolonged period. In thismanner not only the single impulses are made to cause a perceptiblesignal but it is also achieved that whenever the sequence of the lightflashes of the transmitter impulse lamp is faster than one flash duringeach second, the signal-giving lamps 22, 37 lights or/ and the buzzer 23sounds continuously. during the whole onperiod of the light flashtransmitter.

In order that it is readily accessible for the light flashes transmittedfrom a car following behind, the receiver, if constructed as one unit,.must be located at the rear end portion, for instance, of a motor truckor the like. Here, as a rule, it is'exposed to the weather and otherunfavorable influences. To better protect the receiver, it is advisableto dispose it in a safe place near the drivers cab and to mount only thepickup lens and the phototube at the rear of the vehicle. The connectionbetween such pickup unit, which serves actually as a light antenna, andthe receiver proper is, according to this invention, in the form of acable of low capacitance in order to prevent weakening of the incomingsignals as much as possible. The phototube 1, as shown in Fig. 3, ishere placed within the focal length of an optical system, preferablyalens system 31, in such a way that the phototube can receive the lightfrom a relatively large space angle toward the rear. In a practicalembodiment the phototube is mounted in a Searchlight-shaped housinghaving a suitable converging lens system as front covering. Such a unitcan be made very sturdy and can be readily mounted by means of a supportwhich may be adjustable in height and flexible in such manner that itwill yield when striking an obstruction Without being damaged. Asuitable cable for the connection to the receiver is an antenna cable 36as used in auto-radio installation. Such cable has generally acapacitance as low' as 12 micromicrofarads per meter, which is lowenough to not materially impair the sensitivity of the light flashreceiver.

Where no loss of incoming signal strength can be tolerated, it is, inaccordance with this invention (Fig; 4),

possible to insert adjacent to the phototube 1 a pulse formers havingcores of laminated material, which have the advantage of transmittingsuch single impulses in a considerably broader frequency band thanso-called highfrequency transformers with high-permeable cores. Thissurprising effect is due to the fact that extremely short impulses causemagnetization of the iron at one flank of the hysteresis loop only. Thusthe magnetization is merely pushed upward, whereas return to normal isleft to take place gradually withoutrapid decay.

For obtaining highest sensitivity of light flash reception it isadvisable to amplify the voltage impulses directly at the phototube insomewhat similar manner as in a condenser microphone. However, theapplication of an electronic tube for this purpose is not practicable,since such tube is rather delicate for the expectable rough serviceconditions and it requires too many connections for the various tubeelements, whichit would be necessary to accommodate in the connectingcable to the pickup unit. For this reason, the present inventioncontemplates to' employ a semi-conductor triode or transistor 35 (Fig.5) placed adjacent to the phototube 1 and designed for amplification ofthe current value of the voltage impulse by operating with batteryvoltage or equivalent as bias Voltage for the transistor. In such anarrangement it is essential to correlate the output impedance of thetransistor and the wave impedance of the connecting cable 36 for mosteficient passing of the signal impulse. For such proper correlation itmay be advisable to interpose in this circuit a pulse transformerbetween the transistor and cable as indicated at 32 (Fig. 5) and in amanner somewhat similar to the arrangement shown in Fig. 4.

In order to prevent disturbances which could be introduced into thereceiver system through the connecting cable from faulty ignitionsystems of passing cars or the like, the conductors of the cable mustpreferably be arranged symmetrically with respect to the phototube andthe whole cable must be provided with a grounded shielding as indicatedin the Figs. 3 to 5.

The power for the receiver, particularly the plate current of about 200volts D.-C., can be supplied by any suitable well-known means. It isadvisable, as shown in Fig. 1, to convert direct current of, say, 12volts by means of a double-acting vibrator 24 and inverter transformer25. Such double-acting arrangement gives the assurance that the currentsupply is sustained even then when one-half of the vibrator contactsfails. Dry-disk rectifier 26 serves to complete the power source in agenerally known manner. Indicating lamp 27 shows when switch 28 isclosed and the receiver is in operation.

Since the receiver in accordance with this invention amplifies onlyhigh-frequency voltage impulses, it cannot be disturbed by A.-C. humresulting from insufiicient anode voltage filtering. A simple vibratorinterference suppressor, which eliminates high-frequency distrubancesand furthers interference-free car radio reception too, will generallysuflice.

The circuits herewith described are only a few examples of several waysin which this invention may be carried into eflect, and it is obviousthat many changes in circuits and application of elements can be madewithout departing from the spirit and scope of this invention. It is,for instance, possible to employ either less or more amplifier stagesand it is also possible to combine the tubes 6 and 7 to a double tube ina single envelope. A modification is also feasible in which theamplified voltage impulses are applied to raise the potential of acontrol grid of a discharge tube which thereby is made conductive forcurrent actuating the relay 16. Such arrangement, however, has thedrawback that it requires relatively high amplification and it has beenfound that the relaxation method as described is to be preferred sinceit is simpler.

When speaking of light flashes in this specification and particularly inthe claims, this expression is intended to be interpreted as includingalso flashes of light in the infra-red and ultra-violet spectral region.

What I claim is:

l. A receiver for light flash signals of spark-discharge character,comprising a photosensitive device having a photocathode sensitive toradiations near the upper frequency boundary of the visible spectrum andoperative in converting incident spark light flashes into voltageimpulses by photoelectric action, electronic amplifying means coupled tosaid photosensitive device for amplifying said voltage impulses, timingfiltering means interposed between said photosensitive device and saidamplifying means and having a time constant of somewhat longer durationthan the duration of the light flashes to be received, thus beingadapted to pass substantially only voltage impulses resulting from sparklight flashes having a rise in luminous intensity steeper than lux permicrosecond and to prevent the passage of voltage fluctuations caused bylight effects of non-spark-discharge character to said amplifying means,and signal means connected to said amplifying means to be actuated byamplifier voltage impulses.

2. A receiver as in claim 1, wherein said filtering means includes acoupling condenser and a grid leak resistor forming a circuit memberwhose time constant is of somewhat longer duration than the duration ofthe light flashes to be received.

3. A receiver for light flash signals of spark-discharge character,comprising a photosensitive device having a photocathode sensitive toradiations near the upper fre quency boundary of the visible spectrumand operative in converting incident spark light flashes into voltageimpulses by photoelectric action, a network connected to saidphotosensitive device including at least two stages of electronicamplifiers, capacitive coupling means, each including a couplingcondenser and a grid leak resistor, in circuit between saidphotosensitive device and the amplifier stage and between the amplifierstages, the electrical parameters of said coupling means being such thatsubstantially only voltage impulses originating from light flasheshaving a rise in luminous intensity steeper than 100 lux per microsecondare amplified, and signal means connected to the last amplifier stage tobe actuated by said voltage impulses.

4. in a light flash receiver responsive to light flashes of sparkdischarge character, in combination, a photosensitive device having ananode and a photocathode, electronic amplifying means including at leastone amplifier tube having a plate, a cathode and a control electrode,said amplifying means being capable of amplifying voltage impulsesresulting from spark light flashes impinging upon said photocathode andhaving a rise in luminous intensity steeper than 100 lux permicrosecond, coupling means interposed in circuit between saidphotosensitive device and said amplifier tube, said coupling meanscomprising a condenser connected between said anode and said controlelectrode, a ground connection for said photocathode, a grid leakresistor joined at the junction between said condenser and said controlelectrode, the electrical parameters of said condenser and said gridleak resistor being such that these elements form aresistance-capacitance circuit having a time constant of the order of10- second, a source of D.-C. potential connected to the junction ofsaid anode and said condenser, and output means connected to saidamplifying means. 7

5. A receiver for light flash signals of spark-discharge character,comprising a photosensitive device having a photocathode sensitive toradiations near the upper frequency boundary of the visible spectrum andoperative in converting incident light flashes into voltage impulses byphotoelectric 'action, a network connected to said photosensitive deviceincluding at least two stages ofeleetronic amplifiers, filtering meansinterposed between said photosensitive device and the first stage ofsaid amplifiers including a coupling condenser and a grid leak resistorforming a circuit member adapted to pass substantially only voltageimpulses resulting from spark light flashes and to prevent the passageof voltage fluctuations caused by light effects of non-spark-dischargecharacter to said amplifiers, an anode output circuit in the last stageof said amplifiers, a relay having an energizing winding and firstnormally closed contacts, said energizing winding interposed in saidoutput circuit, an auxiliary power source, a relay having an energizingwinding and second normally closed contacts, circuit'means connectingsaid power source'through said first normally closed contacts of saidfirst-named relay to the energizing winding of said last-named relay forholding said second normally closed contacts in open position, signalingmeans, second circuit means connecting said power source through saidsecond normally closed contacts of said last-named relay to saidsignalling means, whereby the contacts of said first-named relay areopened and the contacts of said last-named relay are released to closewhen a voltage impulse passes said anode output circuit, and a condenserbridging the energizing winding of said last-named relay and beingeffective in slowing the action of said last-named relay for keeping itscontacts continuously closed when said first-named relay operates at ahigher than a predetermined rate. V

6. A light flash receiver responsive to light flashes of spark dischargecharacter substantially as described, comprising in combination aphotosensitive device having a photocathode sensitive to radiationsnear' the upper frequency boundary of the visible spectrum and operativein converting incident spark light flashes into steep-fronted voltageimpulses by photoelectric action, a converging lens in front of saidphotocathode for concentrating incident spark light thereon, a bluelight filter interposed between said lens and photocathode said bluelight filter being adapted to augment the radiation pattern inherent inspark light for which said photocathode is particularly sensitive and toretain substantially red and yellow radiation, electronic amplifyingmeans coupled to said photosensitive device for amplifying said voltageimpulses, filtering means interposed between said photosensitive deviceand said amplifying means adapted to pass substantially onlysteep-fronted voltage impulses converted from spark light flashes, saidfiltering means including a coupling condenser and a grid leak resistorforming a circuit member Whose time constant is of somewhat longerduration than the duration of the spark light flashes to be received,and signaling means connected to said amplifying means to be actuated inresponse to spark light flashes received.

7. A receiver as in claim 1, wherein said electronic amplifying meansinclude a relaxation type oscillator circuit tuned for a predeterminedrelaxation period of suflicient duration to cause rise of said voltageimpulses to adequate intensity for actuating said signal means.

8. A receiver as in claim 1, wherein said relaxation type oscillatorcircuit includes a pair of tubes having each anode, cathode andcontrol'grid, a grid condenser and a grid leak resistor interposed incircuit between said tubes, a signal means energizing means connected tothe anode. of one of said tubes, 'said grid condenser and grid leakresistor being tuned for a time constant substantially equal to saidpredetermined relaxation period.

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